1. Field of the Invention
This invention relates in general to a method and apparatus for electrodialysis, and more particularly, to a method and apparatus for rinsing an electrodialysis electrode by exchanging the anode and cathode fluids while maintaining electrical isolation.
2. Description of the Prior Art
Electrodialysis devices consist of a stack or group of cells containing alternating, concentrating and diluting chambers separated by ion exchange membranes. At either end of the stack there are provided electrode compartments; one compartment containing an anode and another compartment containing a cathode. The compartments or cells may be arranged either horizontally or vertically and generally take the form of a stack similar to a filter press. There are, however, cells of an open construction called unit cell construction.
Depending upon the concentration and chemical composition of the feed solution, the electrodes can be rinsed either with a portion of the feed solution or a portion of the concentrated solution thereby making use of the fluid pumps which provide the stack with the fluids to be treated. The pumps provide circulation for the fluids which rinse the two electrode compartments. This arrangement, while outwardly simple, has inherent problems. Such a system permits an electrically conductive path, other than through the device itself, to connect both electrode compartments. Although long passages of small cross sections are provided, this results in a leakage of current through the manifolds, which reduces the efficiency of the electrodialysis device. Sometimes acids or other chemicals are introduced into one or both of the electrical compartments to prevent unwanted reactions, such as scaling or precipitation or the formation of a corrosive gas.
It has also been known to isolate the anode and cathode compartments to prevent unwanted reactions, such as the deposition of metal or the oxidation of a product. By providing anode and cathode streams separate from the main body of fluid to be electrodialyzed, these reactions may be prevented. If the streams for the anode and cathode compartments are kept separate, then a base may be needed and can be added to the anode stream and if needed an acid added to the cathode stream to retain ion contact and prevent pH change. This was generally circumvented in the past by using a common reservoir for both the anode and cathode streams and utilizing a pump to convey the fluids to both the anode and cathode compartment from which it was returned to the reservoir and recirculated. In this manner, the ionic content of both the anode and cathode compartments remained constant. However, in this system, shunt current leaked through the fluid passages and circumvented the electrodialysis device causing an inefficiency in the device. In addition, pumping power was required to produce the circulation.
It has also been known, as for example in U.S. Pat. No. 3,179,583, to make use of the natural circulation caused by the gas generated in the electrode compartment to circulate this fluid. This patent, however, and U.S. Pat. No. 3,203,887 required the use of chemical addition in the form of solid material in a container or in the electrode compartment itself, which is consumed in the electrode process. In U.S. Pat. No. 3,179,583, the same fluid is circulated from the electrode chamber to the reservoir and returns from the reservoir to the chamber.
It has also been known to use polarity reversal in electrodialysis stacks to prevent fowling of membranes and scaling of membranes and electrodes. With current reversal in the past, one of the compartments adjacent the electrode compartment would contain a concentrated solution or liquid and could permit the diffusion of coions through the membrane to deposit on the electrodes or contaminate the electrode compartment.